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(Uncomfortable) conclusions Reactions within the ice matrix seem to be accurately modeled using aqueous parameters (if the correct concentrations are known) Photolysis at the air-ice surface shows different kinetics from that within the ice matrix Heterogeneous reaction kinetics at the air-ice interface may be quite different (or not!!) from those in the ice matrix Exclusion of salts to the air-ice interface may be different from bulk thermochemical predictions The air-ice interface presents a very different solvating environment from the liquid-air interface

4
I will discuss three kinds of experiment Photolysis experiments using ice samples Bimolecular reactions using ice samples Exclusion of solutes and nature of the ice surface

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The QLL Ice matrix Potentially two regions of ice where reactions can occur Reactivity may not be similar in both regions nm scale mm scale In most laboratory experiments, reagents are frozen from solution and samples are melted prior to analysis Often the kinetics may be well predicted from aqueous-phase results Are kinetics measured in bulk ice indicative of reactivity in the QLL? Where does atmospheric ice chemistry occur?

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In aqueous solution On ice A photolysis rate enhancement is observed for harmine on the ice surface as well. But on frozen salt solutions the rate reverts to that seen on the water surface Exclusion of salts during freezing creates an aqueous brine layer at the surface T.F. Kahan et al., Atmos. Chem. Phys., 10, 10917-10922 (2010).

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Does the absorption spectrum and/or the photolysis quantum yield change in the QLL? Naphthalene Emission Ice Water Naphthalene fluorescence in hexanes at 77 K Kawakubo et al. J. Phys. Soc. Japan 1966 21: 1469 Red-shifts in emission spectra on ice indicate self-association: -This is observed for naphthalene, anthracene, phenanthrene, benzene and phenol... Whether aromatic is frozen from solution or deposited from the gas phase and at all concentrations studied

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Molecular dynamics simulations show that aromatics on ice surfaces are not as well solvated by the water molecules present there as on the liquid surface due to the fewer free OH at ice surface. This feature is observed also in the Raman spectrum of surface water vs. ice. Thus the aromatics tend to self-associate at the ice surface to lower their energies there.

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(Uncomfortable) conclusions Reactions within the ice matrix seem to be accurately modeled using aqueous parameters (if the correct concentrations are known) Photolysis at the air-ice surface shows different kinetics from that within the ice matrix Heterogeneous reaction kinetics at the air-ice interface may be quite different (or not!!) from those in the ice matrix Exclusion of salts to the air-ice interface may be different from bulk thermochemical predictions The air-ice interface presents a very different solvating environment from the liquid-air interface